Flash memory is a type of non-volatile memory (NVM) used extensively as secondary storage and long-term persistent storage of electronic data. It is also widely used to store firmware of computers (e.g., the basic input-output operating system (BIOS) of personal computers) and other electronic devices. In addition to being non-volatile, Flash memory is electrically re-writable and requires no moving parts. These attributes have made Flash memory popular for use in portable and battery-powered electronic devices, such as tablet and notebook computers, cell phones, smart phones, personal digital assistants, digital audio players and digital cameras.
Increased processing capability and sophistication of computers and other electronic devices in recent years has led to an increase in demand for higher-capacity Flash memory. To fulfill this demand, Flash memory manufacturers have increased capacity by scaling down the dimensions of the individual memory elements of the Flash memory so that a higher density of memory elements can be formed per given area of a memory array.
The memory elements in Flash memory comprise floating gate transistors formed in a semiconducting material. Each floating gate transistor has a floating gate disposed over a thin tunnel dielectric layer between the drain and source of the transistor. The floating gate transistor is programmed by injecting charge carriers (i.e., electrons) through the thin tunnel dielectric layer and into the floating gate. It is erased by removing charge carriers from the floating gate through the thin tunnel dielectric layer by a process known as quantum tunneling. Only so many of these program and erase (P/E) cycles can be performed before the thin tunnel dielectric layer wears out and the floating gate transistor is no longer able to reliably store charge. The number of P/E cycles that floating gate transistors can endure decreases with scaling, and in recent years there has been shown to be a fundamental limit to the extent to which floating gate transistors can be scaled without suffering data retention problems. Further, a Flash memory cell requires at least three terminals to access the memory cell for a data operation (e.g., a P/E cycle or a read operation). Moreover, Flash memory requires a Flash operating system (Flash OS) and requires an erase operation (e.g., a block erase operation) prior to a write operation, thereby increasing write latency times for write operations.
Alternative NVM technologies that avoid the scaling limits of Flash memory have been proposed. Some of these alternative NVM technologies have shown promise. However, various challenges exist to combining the memory elements of these alternative technologies in a high-capacity memory array.
It would be desirable, therefore, to have high-capacity, re-writable, non-volatile two-terminal cross-point memory arrays that are based on alternative NVM technologies and which avoid the scaling limits and other limitations associated with Flash memory, such as an erase operation prior to a write operation and requiring more than two-terminals to access a memory cell for a data operation.